The present invention relates to a new and improved construction of weighing apparatus for continuously weighing a throughpassing layer of fiber material in a metering device of a spinning preparatory machine, said apparatus being of the type equipped with two pairs of rolls arranged at a distance from one another for supplying the layer of fiber material to and for taking-off the layer of fiber material from a measuring zone disposed between the pairs of rolls, and two weighing plates, each of which is pivotable about an axle and connected with a measuring signal transmitter the signal of which depends upon the pivoting angle of the plates.
The importance of the influence of the uniformity of a layer of fiber material supplied, in the form of a web or fiber layers, for instance to an opening or blending machine or a similar preparatory machine or to a card, upon the quality of not only the product directly delivered by such machines but also a desired end product fabricated from the processed fiber material is well known in the art. The variances, for example, in the quantity or the weight respectively, of a layer of fiber material to be supplied to the machines or weight differences between layers of fiber materials to be blended can not only cause disturbances in the blending ratio, but also in particular can bring about variable yarn counts in a yarn or similar products produced from the web or the fiber layers.
In Swiss Pat. No. 490,526 there is disclosed a weighing device which continuously meters a layer of fiber material which is continuously supplied to a a metering device and delivers the same at a constant production rate to a spinning preparatory machine. In this prior art device a layer of fiber material is supplied by a pair of feed rolls and is transported by a pair of transporting rolls through a measuring zone and after being measured is taken-off by a pair of delivery rolls. The aforementioned pairs of rolls are driven by a drive mechanism at speeds, the ratios of which relative to one another remain constant. In the measuring zone there is arranged a weighing device which measures the fiber weight and by means of a control device which is connected with the weighing device and the drive mechanism there is controlled the speed of the pairs of rolls.
The known weighing device located in the measuring zone between the pair of transporting rolls and the pair of delivery rolls consists of two weighing plates, the ends of which facing the respective pairs of rolls are arranged to be upwardly and downwardly pivotable about a hinge or pivot and the ends of which facing each other and located at approximately the middle of the measuring zone are downwardly angled or flexed. These downwardly angled ends are supported by the upper part of a measuring cell or box which through the agency of control devices controls the drive mechanism of the aforementioned pairs of rolls. If the weight of the layer of fibers deviates from the desired weight the weighing plates correspondingly pivot upward or downward in such a manner that the measuring cell indicates a value differing from a pre-set reference or desired value. This value measured at each moment is delivered to the control devices which correspondingly adapt or change the speed of the drive motor of the afore-mentioned pairs of rolls and thus change the quantity of fiber material which is supplied, transported through the measuring zone and delivered.
In this state-of-the-art weighing device it has been found that the friction forces exerted by the fiber layers gliding across the weighing plates are of considerable influence. These forces act in addition to the weight and irregularly influence the measuring process of the measuring cell and thus cause an undesirable and falsified control of the drive motor.